1. Field of the Invention
This invention relates to a wipe pattern generating apparatus employed for producing special image effects.
2. Prior Art
There is known a display converting technique in which an auxiliary image is displayed within a so-called wipe pattern having one of various contours, such as rectangular or circular contour, on the main image displayed on the display screen of a television receiver.
The conventional wipe pattern generator adapted for generating a wipe pattern employed in the display converting technique is arranged as shown in FIG. 1.
In the wipe pattern generating apparatus shown in FIG. 1, when a wipe pattern of a polygon, for example, a regular triangle, is to be produced, three plane generators 1, 2 and 3 generate three planes corresponding to the three sides other than the bottom side of a regular triangular pyramid. A non-additive mixing (NAM) circuit 4 processes the three planes by a so-called NAM processing and takes a minimum value to form a regular triangular pyramid as shown in FIG. 2. That is, rectangular coordinate data (x, y, h) within the three-dimensional space of the respective planes of the regular triangular pyramid are obtained by the NAM circuit 4.
On the other hand, serrated wave generating circuit 5 and 6 generate serrated waves H and V with the aid of horizontal sync signals HD and vertical sync signals VD. A coordinate converting circuit 7 generates rectangular coordinate (x, y) indicating the pixel position on the screen with the aid of the data HD and VD.
Referring to FIG. 2, an operating circuit 8 calculates the height h.sub.1 of the regular triangular pyramid for rectangular coordinate data (x.sub.1, y.sub.1) from the coordinate conversion circuit 7, with the aid of rectangular coordinate data (x.sub.1, y.sub.1, h.sub.1) of each of the planes of the regular triangular pyramid from the NAM circuit 54.
In a comparator 9, the height h.sub.1 of the regular triangular pyramid for the rectangular coordinate data (x.sub.1, y.sub.1) from the operating circuit 8 is compared with a fader level h.sub.th. A signal which becomes "1" of h.sub.1 .gtoreq.h.sub.th and "0" otherwise is outputted from the comparator circuit 9 as a wipe pattern W. Thus the shape of the sectional surface at a height shown by the fader level h.sub.th proves to be the wipe pattern W. By changing the fader level h.sub.th, the size of the wipe pattern in the form of a regular triangle can be changed to achieve the above mentioned display conversion.
Meanwhile, with the above mentioned wipe pattern generator, three plane generators 1, 2 and 3 are necessary to generate the wipe pattern of the regular triangle, as explained above, while four plane generators are necessary to generate the wipe pattern of a square. That is, an N number of plane generators are necessary to generate an N-gonal wipe pattern. However, an increase in the number of sides of the regular polygon results an increase in the number of hardware items.
There has also been known a wipe pattern generator as disclosed for example in U.S. Pat. No. 4,954,898, in which data indicating the contour of a wipe pattern of a predetermined shape expressed by polar coordinates, referred to hereinafter as polar coordinate data, are previously stored in a memory, the rectangular coordinate data indicating the positions of the pixels on the screen are converted into polar coordinate data, the radius vector of the contour of the wipe pattern stored in the memory, referred to hereinafter as distance data, are read out using the deflection angle of the polar coordinate data, referred to hereinafter as angle data, and the distance data thus read out are compared to the distance data of the polar coordinate data of the pixels to generate the wipe pattern. That is, the wipe pattern is generated on the basis of judgment as to whether the positions of the pixels on the screen are within or outside of the contour of the wipe pattern stored in the memory. The distance data read out from the memory are also multiplied by a coefficient data to change the wipe pattern size, thereby converting the display on the screen.
However, with the above described wipe pattern generator, for generating the wipe pattern of a plurality of shapes, such as regular triangular, square or heart shapes, it is necessary to provide a memory for storing polar coordinate data indicating the contours of as many different kinds of wipe patterns. However, if the number of kinds of the wipe patterns to be generated is increased, it is necessary to provide a memory of a larger storage capacity.
In addition, with the above described conventional wipe pattern generator employing the plane generators, it is only possible to generate a wipe pattern having a linear contour.